[wip][performance] SquashFS: Use unbuffered pread when possible to ge…

…t file blocks

core/ratarmountcore/SquashFSMountSource.py

@@ -75,7 +75,7 @@ def _dir_scan(self, start_block, offset):
 from .MountSource import FileInfo, MountSource
 from .SQLiteIndex import SQLiteIndex, SQLiteIndexedTarUserData
 from .SQLiteIndexMountSource import SQLiteIndexMountSource
-from .utils import InvalidIndexError, overrides
+from .utils import InvalidIndexError, openPreadable, overrides
 
 
 class IsalZlibDecompressor(Compressor):
@@ -376,6 +376,20 @@ def _recursive_inodes_iterator(self, directory):  # -> PySquashfsImage.file.File
                 cls = PySquashfsImage.filetype[entry["type"]]
                 yield self._join_inode_offset(start_block, offset), cls(self, inode, entry["name"], directory)
 
+    def _read_data_block(self, start, size):
+        print("[_read_data_block]")
+        compressedSizeBlock = PySquashfsImage.SQUASHFS_COMPRESSED_SIZE_BLOCK(size)
+        offset = self._offset + start
+        if hasattr(self._fd, 'pread'):
+            data = self._fd.pread(compressedSizeBlock, offset)
+        else:
+            self._fd.seek(offset)
+            data = self._fd.read(compressedSizeBlock)
+        if PySquashfsImage.SQUASHFS_COMPRESSED_BLOCK(size):
+            return self._comp.uncompress(data, compressedSizeBlock, self._sblk.block_size)
+        else:
+            return data
+
     @property
     def _root(self):
         if self._real_root is None:
@@ -408,7 +422,11 @@ def __init__(
         **options
         # fmt: on
     ) -> None:
-        self.rawFileObject = open(fileOrPath, 'rb') if isinstance(fileOrPath, str) else fileOrPath
+        # Beware: We should not turn of file buffering because PySquashfsImage does LOTS of very small reads to
+        #         read lists of integers to parse the SquashFS superblock. It would be nice to buffer this
+        #         on the parser side, but for now, leave the file object buffered and instead try to use unbuffered
+        #         reads for file accesses only.
+        self.rawFileObject = openPreadable(fileOrPath) if isinstance(fileOrPath, str) else fileOrPath
         self.rawFileObject.seek(0)
         offset = findSquashFSOffset(self.rawFileObject)
         if offset < 0: